The present invention relates generally to the continuous casting of molten metal and more particularly to the detection of break-outs during continuous casting.
In a continuous casting process, molten metal is continuously introduced into the top of a vertically disposed, liquid cooled, metal mold having open upper and lower ends. The metal descends through the mold, and partially solidified metal is continuously withdrawn from the bottom of the mold. More particularly, as molten metal descends through the mold, the metal in contact with the interior surface of the cooled mold is chilled to form a cast metal shell surrounding an interior of molten metal, and this is normally the form of the metal when it is withdrawn from the bottom of the mold. Conventional expedients are employed at the start of the casting operation to retain the metal within the mold until after there is solidification at the bottom of the shell.
As the shell descends through the mold, it thickens. During the casting process, a hot spot develops in the mold wall, slightly below the top surface of the molten metal in the mold, and that surface is typically maintained near the upper end of the mold. During initiation of a sticker or hanger-type break-out, as the cast metal shell descends through the mold, the hot spot similarly descends, at a slower rate, causing a gap in or thinning of the cast metal shell at the location of the descending hot spot. When the hot spot reaches the lower open end of the mold, a break-out of molten metal occurs. Break-outs are dangerous and wasteful.
There are two predominant types of break-outs: hangers and stickers. Hanger-type break-outs are caused by molten metal overflowing the top of the mold. Sticker-type break-outs are initiated when the upper part of the shell, or a portion thereof, gets stuck to the mold wall and tears apart from the rest of the descending shell.
A more detailed discussion of hot spots and break-outs and the considerations involved with respect thereto is contained in a paper by the present inventors entitled "An Investigation Of Sticker And Hanger Breakouts", 4th International Conference on Continuous Casting, Brussels, May 17-19, 1988, pp. 668-681, and the disclosure thereof is incorporated herein by reference.
In a typical commercial, vertically disposed, continuous casting mold, cooling liquid is circulated through vertically disposed channels in the mold sidewalls. In addition, a series of temperature sensors in the form of thermocouples are embedded within the side walls of the mold, at vertically spaced locations therein, to measure the temperature at each of these vertically spaced locations. These temperature measurements are indicative of the relative temperature of the metal shell within the mold at a respective vertical location on the mold.
There is a prior art procedure for predicting the likelihood of a molten metal break-out at the lower open end of the continuous casting mold. This procedure employs the arrangement of mold wall thermocouples described in the previous paragraph and utilizes, from each of several vertically spaced thermocouples, e.g., three thermocouples, a continuous temperature measurement which is plotted on a graph on which the vertical coordinate is temperature and the horizontal coordinate is time. The temperature versus time curves for the several thermocouples are plotted on the same graph. In a normal casting operation, where there is no danger of a break-out, the temperature reading should decrease progressively in descending order among the thermocouples. When a thermocouple near the top of the mold measures a brief rise followed by a drop in temperature, with time; and when this temperature behavior is repeated at each of the lower thermocouples, in descending order, sequentially, it means that there is a descending hot spot and that there is a danger of a break-out unless corrective action is taken. A typical corrective action is to slow or stop the withdrawal of the continuously cast shell from the mold, as this gives the metal in the shell an opportunity to freeze and/or thicken at the location of the hot spot.
A more detailed description of the break-out predicting procedure described above is contained in a paper by Tsuneoka, et al., "Measurement and Control System of Solidification in Continuous Casting Mold", Steelmaking Conference Proceedings, AIME, 1985, pp. 3-10, particularly at pp. 3-5.
A drawback to relying upon an arrangement of thermocouples embedded in the walls of the continuous casting mold, for predicting a break-out, is that these thermocouples are subjected to extremely severe service conditions and require frequent servicing or replacement. For that reason, they cannot always be relied upon to provide an accurate indication of the temperature conditions within the mold at all levels thereof, on a continuous basis.
Other break-out predicting devices and procedures, based on variations, with time, of mold friction or overall mold heat transfer rate, are not sufficiently reliable in predicting break-outs, and therefore should not be used for that purpose.